[dm-devel] [PATCH v2] block: use gcd() to fix chunk_sectors limit stacking

[Mike Snitzer]

On Thu, Dec 03 2020 at  8:45pm -0500,
Ming Lei <ming.lei at redhat.com> wrote:

> On Thu, Dec 03, 2020 at 08:27:38AM -0800, Keith Busch wrote:
> > On Thu, Dec 03, 2020 at 09:33:59AM -0500, Mike Snitzer wrote:
> > > On Wed, Dec 02 2020 at 10:26pm -0500,
> > > Ming Lei <ming.lei at redhat.com> wrote:
> > > 
> > > > I understand it isn't related with correctness, because the underlying
> > > > queue can split by its own chunk_sectors limit further. So is the issue
> > > > too many further-splitting on queue with chunk_sectors 8? then CPU
> > > > utilization is increased? Or other issue?
> > > 
> > > No, this is all about correctness.
> > > 
> > > Seems you're confining the definition of the possible stacking so that
> > > the top-level device isn't allowed to have its own hard requirements on
> > > IO sizes it sends to its internal implementation.  Just because the
> > > underlying device can split further doesn't mean that the top-level
> > > virtual driver can service larger IO sizes (not if the chunk_sectors
> > > stacking throws away the hint the virtual driver provided because it
> > > used lcm_not_zero).
> > 
> > I may be missing something obvious here, but if the lower layers split
> > to their desired boundary already, why does this limit need to stack?
> > Won't it also work if each layer sets their desired chunk_sectors
> > without considering their lower layers? The commit that initially
> > stacked chunk_sectors doesn't provide any explanation.
> 
> There could be several reasons:
> 
> 1) some limits have to be stacking, such as logical block size, because
> lower layering may not handle un-aligned IO
> 
> 2) performance reason, if every limits are stacked on topmost layer, in
> theory IO just needs to be splitted in top layer, and not need to be
> splitted further from all lower layer at all. But there should be exceptions
> in unusual case, such as, lowering queue's limit changed after the stacking
> limits are setup.
> 
> 3) history reason, bio splitting is much younger than stacking queue
> limits.
> 
> Maybe others?

Hannes didn't actually justify why he added chunk_sectors to
blk_stack_limits:

commit 987b3b26eb7b19960160505faf9b2f50ae77e14d
Author: Hannes Reinecke <hare at suse.de>
Date:   Tue Oct 18 15:40:31 2016 +0900

    block: update chunk_sectors in blk_stack_limits()

    Signed-off-by: Hannes Reinecke <hare at suse.com>
    Signed-off-by: Damien Le Moal <damien.lemoal at hgst.com>
    Reviewed-by: Christoph Hellwig <hch at lst.de>
    Reviewed-by: Martin K. Petersen <martin.petersen at oracle.com>
    Reviewed-by: Shaun Tancheff <shaun.tancheff at seagate.com>
    Tested-by: Shaun Tancheff <shaun.tancheff at seagate.com>
    Signed-off-by: Jens Axboe <axboe at fb.com>

Likely felt it needed for zoned or NVMe devices.. dunno.

But given how we now have a model where block core, or DM core, will
split as needed I don't think normalizing chunk_sectors (to the degree
full use of blk_stack_limits does) and than using it as basis for
splitting makes a lot of sense.

Mike